Jeroen Demeyer on Wed, 03 Jun 2015 16:05:28 +0200


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Struggling with ellisogeny() and variable priorities 

This works:
gp> E = ellinit([Mod(0, y^2 + 1), Mod(0, y^2 + 1), Mod(0, y^2 + 1), Mod(1, y^2 + 1), Mod(0, y^2 + 1)]) 
gp> isog = factor(elldivpol(E,37))[1,1];
gp> yy = varhigher("yy")
%2 = yy
gp> xx = varhigher("xx")
%3 = xx
gp> ellisogeny(E, isog, 0, xx, yy);


But this (which should be equivalent, right?) does not:
gp> E = ellinit([Mod(0, y^2 + 1), Mod(0, y^2 + 1), Mod(0, y^2 + 1), Mod(1, y^2 + 1), Mod(0, y^2 + 1)]) 
gp> isog = factor(elldivpol(E,37))[1,1];
gp> yy
%2 = yy
gp> xx
%3 = xx
gp> ellisogeny(E, isog, 0, xx, yy)
  ***   at top-level: ellisogeny(E,isog,0,
  ***                 ^--------------------
  *** ellisogeny: incorrect priority in ellisogeny: variable xx <= yy